This invention relates to apparatus for transport and storage of semiconductor substrates.
A front opening unified pod (FOUP) is a container for transporting and storing semiconductor substrates. A FOUP combines a cassette and a boat box for holding cassettes. It has shelves for holding substrates, an outer box-shaped pod body having five sides, and a door for sealing the pod body""s sixth side. The frame is slightly larger, both in terms of width and length, than the pod body. The door is bolted into the frame, and the frame is fused to the pod body. The pod body, shelves, and door are made of plastic. Typically, to open the door, keys need to be inserted into a lock assembly in the door.
In use, a single FOUP is placed in a loading station of a semiconductor processing tool, flush against a tool door having two keys. The tool door, attached to an arm, opens the FOUP door by turning the keys in the FOUP door, thus opening bolts that attach the FOUP door to the FOUP body. The tool door pulls on the FOUP door with a vacuum, pulls the FOUP door away from the FOUP pod body, and lowers the FOUP door, thereby exposing wafers inside the FOUP. After a wafer is removed from the FOUP for processing and is processed, the wafer is returned to the FOUP.
The FOUP door cannot be opened without a tool door, unless one has two keys manufactured, inserts the keys into the FOUP door by hand, turns the keys simultaneously, and pulls the door off. This option is undesirable because opening the FOUP door outside a FOUP station can expose the wafers inside the FOUP to a relatively unclean environment.
The door of a FOUP is generally opened in ultra-clean system environments, e.g. Class 1, to avoid contaminating wafers inside the FOUP. Sometimes, however, wafers may be exposed to less clean environments. For example, during a lithography process, a wafer may be removed from a FOUP in a Class 1 environment and fed by a track to a scanner. The wafer is processed and returned to the FOUP pod via the track. If, however, the wafer is rejected by the scanner, it is not returned to the FOUP. Instead, it is transferred to an open wafer cassette, e.g., a Crystalpak(copyright), with other rejected wafers. A Crystalpak(copyright), manufactured by Entegris, Inc., is a plastic cassette in which wafers are shipped by a wafer manufacturer. A Crystalpak(copyright) has 13 slots for holding 13 300 mm wafers and has a height of 18 centimeters (cm). On the other hand, a FOUP currently used in industry has 25 slots for holding 25 300 mm wafers and is twice the height of a Crystalpak(copyright), e.g., 36 cm. A Crystalpak(copyright) can be placed inside a scanner, and rejected wafers can be moved into the Crystalpak(copyright). The scanner""s loading arm is stationery and the wafer carrier moves during loading and unloading. The configuration does not provide sufficient range of motion for loading and unloading wafers to/from a FOUP, but it is sufficient for, e.g., a Crystalpak(copyright). The wafer cassette is then removed from the scanner, and the wafers in the boat are transferred to another FOUP. The wafers in the wafer boat are thus exposed to a less clean atmosphere than that of the processing equipment, such as Class 10 instead of Class 1. This exposure can contaminate the wafers, increasing defect densities. The wafers in the system environment cannot be placed directly into a second FOUP if the system has only one port for a FOUP. Further, a FOUP cannot be placed inside the processing tool because of constraints imposed by the method of opening the front door, such as the requirement that a tool door be used for opening the FOUP door. Finally, a FOUP may not physically fit inside a tool, even in the FOUP""s closed position.